Project Summary Neural stem cell/progenitors cells give rise to mature neurons, astrocytes and oligodendrocytes throughout life. However, neurogenesis rapidly declines during aging and the mechanism for age-dependent neural stem cell dysfunction is not clear. The ventricular-subventricular zone (V- SVZ), lining the lateral ventricle, is home to the largest pool of neural stem cells in the murine brain. We have found that microglia, the innate immune cells of the brain, are prominently positioned throughout the young and aged V-SVZ niche. During aging, microglia undergo a morphological and phenotypical shift from a resting state to a pro-inflammatory state. Preliminary data suggest that secreted molecules from young microglia support proliferation and neuronal differentiation in vitro. In contrast, microglia isolated from aged mice appear to lose this influence on proliferation in vitro. Together, these data suggest microglia play a critical age- dependent role in regulating neurogenesis. Although microglia are known to be important in phagocytosis of neuroblasts, their influence on type B neural stem cells, type C transit amplifying cells and niche cytoarchitecture is essentially unknown. Using 3-dimensional image analysis of niche cytoarchitecture and flow activated cell sorting (FACS), we will test the hypothesis that microglia have opposing roles in the young and aged neurogenic niche. In aim 1, we will pharmacologically and genetically deplete microglia from the young neurogenic niche and interrogate the impact on neurogenesis, Type B, C and neuroblast cell survival as well as number, proliferation and position near the vascular compartment. In aim 2, we will use heterochronic infusion of secreted molecules from young and aged microglia to directly test if these molecules have opposing roles in neurogenesis. In aim 3, we will test if mitigating the inflammatory phenotype of aged microglia restores neural stem/progenitor cell function. We will also explore novel molecular candidates for microglia derived secreted molecules that support or hinder neurogenesis. Understanding how the microglia activation state regulates neurogenesis will have far-reaching consequences. It is widely accepted that V-SVZ neural stem cells and their progeny contribute to brain repair. Thus, understanding how microglia contribute to neurogenesis during tissue homeostasis and aging will not only further our basic understanding of neurogenesis but will help in the eventual goal of using neural stem/progenitor cells in brain therapeutics.